Vibration tamper

ABSTRACT

A tamping machine including a motor, a crank driven by said motor over an appropriate mechanical transmission, a linkage connecting the crank to a vertically reciprocating plunger disc, a tamping foot disposed beneath the plunger disc, a housing encasing the crank, linkage and the upper end of the tamping foot, a first set of springs for urging the tamping foot to move in response to movement of the plunger disc, and a second set of springs for damping the shock forces created within the machine when the tamping foot strikes the material to be compacted. The second set of springs eliminates, or at least minimizes, the wear and tear on the drive mechanism of the tamping machine caused by the shock forces by transmitting, directly or indirectly, a substantial portion of such shock forces to the sturdy machine body. The second set of springs are connected between the machine body and the plunger disc and/or tamping foot.

United. States Patent [191 Linz [ VIBRATION TAMPER [75] Inventor: Albert Linz, Hoffnungsthal, Germany [73] Assignee: Boppard er Maschinenbaugesellschal't mbH, (BOMAG) Boppard/Rhine, Germany [22] Filed: Oct. 15, 1971 1 [21] Appl. No.: 189,555

[52] US. Cl. 404/133 [51] Int. Cl. E0lc 19/34 [58] Field of Search 94/49, 48; 404/133 [56] References Cited UNITED STATES PATENTS 3,270,635 9/1966 Kestel 94/49 3,090,286 5/1963 Kestel 94/49 3,308,729 3/1967 Kestel 94/49 3,286,790 11/1966 Kestel 94/49 X 3,630,127 12/1971 Yamato 94/49 3,538,821 11/1970 Baeumers 94/49 3,277,801 10/1966 l-lorvath 94/49 [451 Sept. 4, 1973 Miller 94/49 3,236,164 2/1966 2,845,050 7/1958 Wacker 94/49 X 3,162,102 12/1964 Juneau 94/49 Primary Examiner-Nile C. Byers, .lr. Att0meyAndrew .1. Beck et al.

[57] ABSTRACT linkage and the upper end of the tamping foot, a first set of springs for urging the tamping foot to move in response to movement of the plunger disc, and a second set of springs for damping the shock forces, created within the machine when the tamping foot strikes the material to be compacted. The second set of springs eliminates, or at least minimizes, the wear and tear on the drive mechanism of the tamping machine caused by the shock forces by transmitting, directly or indirectly, a substantial portion of such shock forces to the sturdy machine body. The second set of springs are connected between the machine body and the plunger disc and/or tamping foot.

housing.

Conventional vibration tampers of this general type come in various or embodiments. For example, there a tamping unit in which a motor driven crank drive is connected by means of a rod to a frame consisting of individual rods moving through sliding sleeves, and the frame is closed 011' on the top and bottom sides by cross bars. Inside the frame are two compression springs one on top of the other. Between these springs a cross bar is connected to the compressing tool, so that the upper and lower springs rest against the frame at one end and against the compressing tool at the other. There is also a tamping unit having a guide cylinder open at the top which encloses a cylindrical part of the drive housing and is provided with several guide sleeves on the outside at the bottom. Several posts are attached to the rarnmer part, each post reaching a guide sleeve and supporting two springs located one on top of the other. The guide sleeve is located between these springs so that the lower spring rests against the compressing tool at one end and against the guide sleeve at the other, while the upper spring likewise rests againstthe guide sleeve at one end and against the retainer attached to the upper end of the posts at the other.

In observing the flux of forces in these devices, it can be seen that the equilibrium of forces results from the springs located between the compressing tool and the plunger disc. The transfer of energy from the motor to the plunger disc, on the other hand, is achieved entirely through the guide rod and the gear unit. The resulting forces from the unequally accelerated movements of the driving member and the transmission assembly, caused by their differing masses, must be absorbed fully by the gears and the driving members. The result is that these parts of the device are under considerable strain, since the compressing tool frequently encounters impediments with increased resistance during operation; the amplitude of downward vibration action of the device is forcibly reduced, whereby a parallel shift in the neutral oscillation position of all masses is caused so that the vertical accelerating forces increase suddenly. While these forces are absorbed between the compressing tool and the plunger disc by interposing springs, the forces between the plunger disc andthe driving member must be entirely absorbed by the individual drive parts. The intermittently increased mass forces, however, cause extraordinary gear flank and bearing pressure and consequently great wear and tear on the drive parts. In equipment built on this principle the parts under special stress must be frequently checked and replaced, whereby considerable costs for maintenance and work stoppage can be the result.

The purpose underlying the invention thus consists of eliminating the disadvantages described above. The effect on the drive parts of hard shocks caused by intermittent jarring blows, which are not avoidable them- The solution of the problem is based on the idea of providing an alternative means for the transmission of force which would relieve the drive member from vertically upward shocks, so that the intermittent jarring shocks would be transmitted directly into the housing by partially bypassing the drive parts.

According to the invention the problem is solved by connecting the compressing tool to the housing with springs. An advantageous development of this vibration tamper is achieved by connecting the guide rods firmly to the compressing tool.

One variation of the invention consists in connecting the working tool indirectly with the housing by springs. This is made possible by placing the guide rods in the sliding sleeves of the housing in such a way that they can be shified vertically, and by placing springs around the guide rods between the compressing tool and the plunger disc, and additional springs between the plunger disc and the sliding sleeve, as well as springs between the sliding sleeve and the retainer fastened to the guide rods. Another advantageous feature of this device consists of fastening a cylindrical part containing a guide rod which is attachedto the compressing tool and has a retainer at its end at the bottom of the plunger disc. Between the" retainer and the bottom of the cylindrical part a prestressed compression spring is installed,'concentrically located around the guide rod.

According to another variation of the vibration tamper it is possible to connect the compressing tool to the housing of the device directly by springs. This is done by installing additional springs betweenthe compressingtool and thesliding sleeve springs in the housing, and between the sliding sleeves and the retainer attached to the guide rods. One or more springs are also placed between the plunger disc and the compressing tool and between the plunger disc and the upper cover plate of the compressing tool or the retainer attached to the guide rods.

In practice it is advantageous for structural reasons to equip the instant device with prestressed springs located around the guide rods concentrically. In this case each of the guide rods can be placed at the corners of an equilateral triangle.

. Further advantageous features of the instant invention will become apparent from the following description, when construed invharmony with the accompanying drawings, wherein: i

FIG. 1 is a diagrammatic representation of the preferred spring-mass system according to the invention.

FIG. 2 is a diagrammatic representation of a first alternative spring-mass system for the instant device.

FIG. 3 is a schematic view of a design based on the spring-mass system of FIG. 1. r

FIG. 4 is a schematic view of the first alternative spring-mass system of FIG. 2. I

FIG. 5 is a schematic view of another variation based on the spring-mass system of FIG. 2.

FIG. 6 is a schematic view of yet anothqr variation based on the spring-mass system of FIG. 2.,

FIG. 7 is a vertical cross section of the vibration tamper constructed in accordance with the spring-mass system of FIG. 1. v

FIG. 8 is a horizontal cross section across the line VIII-VIII in FIG. 7. 2

FIG. 9 is a vertical cross section of another vibration tamper constructed in accordance with the spring-mass system of FIG. 2.

FIG. is a horizontal cross section at the line X-X in FIG. 9.

A preferred arrangement of the spring-mass system of the device is shown in FIG. 1, in which the letters represent the following:

Ml Mass of the upper part of the device containing the driving motor, the transmission and the housing. M2 Mass of the machine parts moved directly in a straight line in the direction of the axis Y by the tamper drive.

M3 Mass of the working part (compression unit). F1 Groups of springs which connect mass M2 and mass M1 by the action of the compression springs and tension springs.

F2 Groups of springs which connect masses M2 and M3 by the action of the compression springs and tension springs.

K1 Crank drive.

Pl Rod as connecting and transmitting member between M1 and M2.

Masses M1, M2 and M3 are connected with each other by means of suitable guiding elements in such a way that they can perform motions relative to each other and a compressing tool in the direction of the axis Yl-Y1, whereby axes X1, X2 and X3 always remain parallel to each other. In can further be seen from the illustration that mass M1 is larger than M3, and M3 is larger than M2.

If the spring elements F1 are left out of FIG. 1, one obtains the design principle of known compressing equipment in which plunger disc M2 is put in motion by crank drive K1 and connecting rod P1, the up an down movements of which are transmitted to the compressing tool X3 through spring F2; the compressing tool thus starts to vibrate. In this arrangement the equilibrium of forces between M2 and M3 is brought about by spring F2, while between M1 and M2 the force components working in a vertical direction are absorbed entirely by the elements of the crank drive (kl, Pl). Thus if jarring blows occur in actual operation which produce high intermittent inertial forces, these additional great stresses are absorbed entirely by the drive parts (gears, crankshaft, connecting rod, and bearing). Increased gear flank pressure and bearing pressure then develop and cause great wear and tear on the drive parts of such known compressing machines.

In contrast thereto, in the preferred spring-mass system upon which the instant invention is predicated, M1 is cushioned against M2 through the installation of the tension and compression spring F1, so that Fl acts as supporting and connecting spring. If vertically directed shocks occur in this arrangement of the spring-mass system, the springs F1 take over a part of the accelerating forces of the masses M1 and M2 and thereby relieve the drive parts. By means of a suitable choice of the spring constants of the springs Fl, any relief desired can be achieved; it must be taken into consideration, however, that the spring force produced by the springs F l and directed against the ascending motion of the plunger disc is lower than the driving torque available to the crankshaft.

The same effect can be achieved by the alternative spring-mass system shown in FIG. 2. In this arrangement M4 is directly connected to M6 by means of the compression and tension springs F4 (supporting springs) and not indirectly through F1 and F2 as in FIG. 1. M5 is connected to M6 in the usual way, by

means of a tension and compression spring F5. The connecting spring F5 transmits vibratory motion from M5 to M6, while the springs F4 serve to transmit blows directly to the housing while by-passing the driving elements.

FIGS. 5-8 are schematic diagrams views of various arrangements of the vibration tamper. In these examples prestressed compression springs have been used for structural reasons demonstrated in practice, instead of the tension and compression springs used in FIG. 1 and FIG. 2.

In FIG. 3 part 1 of the device part M1 in FIG. 1 is shown with a crank gear 2 and the connecting rod 3. The connecting rod 3 is attached to the plunger disc 4 M2 in FIG. 1 while the compressing tool 5 corresponds to the mass M3 in FIG. 1. On the top side of the compressing tool 5 several guide rods 6 have been inserted; in the sliding sleeves 7 of the upper part of the device 1, and in the sleeve 8 of the plunger disc 4, these are guided so that they can move vertically. The springs 9 between the plunger disc 4 and the compressing tool 5, the springs 10 between part 1 of the device and the plunger disc 4, and the springs 11 between the housing 1 and the retaining plates 12 attached to the upper part of the rods move around'the guide rods 6 concentrically. In this way housing 1, in a state of rest, is supported against the compressing tool 5 by the springs 10, the plunger disc 4, and the springs 9. Under operating conditions, the springs 9 are compressed further as a result of the downstroke of plunger disc 4 as driven by drive gear 2. This causes compressing tool 5 also to move downwards. At the same time springs 10 are relaxed and springs 11 compressed due to the downstroke of guide rods 6. On the upstroke of plunger disc 4, springs 10 are once again compressed as a result of the upward movement caused by thesimultaneous release of springs 9 and 11. In this example, only the ten sion force of springs 11 causesthe upward movement of the compressing tool. In this system the motion of the plunger disc has a direct effect on the motion of the compressing tool only on the downstroke, disregarding the action inertia of the mass of the springs 9.

The difi'erence between the device shown in FIG. 4 and the one in FIG. 3 consists of a cylindrical part 14 attached to the bottom side of plunger disc 4. A guide rod 13, which is firmly connected to the compressing tool and is provided with a retainer plate 16 at its end, extends. into the cylindrical part 14 through an opening 17. The compression spring 15 is located concentrically around the rod 13 between the retainer 16 and the bottom of part 14. in connection with this the upstroke of the plunger disc 4 affects the compressing tool directly so that the spring tension resulting from the up and down motions of the plunger disc 4 exerts a driving force onthe compressing tool 5.

FIG. 5 and FIG. 6 show arrangements of the same device with a spring-mass system like that pictured in FIG. 2. As in the foregoing descriptions, part 20 is also present here, as well as crank drive 21 which moves plunger disc 26 attached to the end of guide rod 23 by means of connecting rod 22. The compressing tool 25 is firmly connected to guiding rods 29 which are in sliding sleeves 31 and are provided with retainer plates 30 at their end. The guiding rods 29 and 48 are guided in sleeves 31 and 50 respectively, so that they can move vertically. Springs 32 and 51 in parts 30 and 31 '(FIG. 5) and 49 and 50 (FIG. 6) respectively, as well as springs 33 and 52 between the sliding sleeves 31 and 50 respectively, and the cover plates 24a and 44a of the compressing tool surround rods 29 and 48 concentrically.

In FIG. 5 the plunger disc attached to the end of the guide rod 23 extends into a cylindrical cavity through the opening 24 of cover plate 24a of the compressing tool. One of the springs placed around the guide rod 23 concentrically is located between the plunger disc 26 and the cover plate 24a. A spring 28 is located between the plunger disc 26 and the bottom of the cavity. In contrast to FIG. 5, FIG. 6 shows a compressing tool having a larger cylindrical cavity which contains two parallel spring systems 53 and 54. Additional guide rods, guiding the plunger disc 46 and surrounded by springs 53 and 54 concentrically are firmly attached to the compressing tool at their lower ends; their upper ends reach the cover plate 440 or are provided with a retainer.

In a state of rest, parts and 40 are held against the compressing tool through springs 33 and 52. While in operation, these parts compress spring 28 or spring 54 on the downstroke of the plunger disc so that the compressing tool also moves downward, and springs 33 and 52 are relaxed while springs 32 and 51 are extended as a result of the simultaneous downward movement of guide rods 29 and 48. In the same way spring 27 and spring 53 are relaxed during the downstroke of the plunger disc. On the upstroke of the plunger disc springs 28 and 54 as well as springs 32 and 51 are relaxed, while springs 27, 53, 33 and 52 are compressed. The result of this arrangement is that the upstroke as well as the downstroke of the plunger discs 26 and 46 act directly on the compressing tools and 45, respectively. The jarring blows are transmitted directly from the compressing tool to the housing through springs 33 and 52. In this way fewer stresses on the gear occur, and a reduction of wear and tear is achieved.

FIGS. 7 and 8 show the vertical and horizontal cross sections of the design of the system according to FIG. 3. The figures are limited to represent those parts of the vibration tamper which are important for the mutual shock absorption of the individual parts. The cylindrical housing contains essentially the following parts: a connecting rod 62, the guide rods 64, a spring retainer 65, the spring groups 66, 68 and 72, a plunger disc 70, and a compressing tool or tamping foot 74.

The drive housing with a motor and the housing 63 together form the upper part of the device of the spring-mass system in FIG. 3, or mass M1 in FIG. 1. Likewise the plunger disc 70 with the sliding sleeves 69, pin 71, and parts of the connecting rod 62 correspond to mass M2, and the tamper or working part 74 with the guide rods 64 and the spring retainers 65 correspond to mass M3 in FIG. I. The rods 64glide in'the sleeves 67 of the housing 63, and in the sleeves 69 of the plunger disc 70. For practical reasons the compressing tool is fitted into the housing so that it slides freely. In this way an accurate vertical motion is achieved through the slideways in the housing itself as well as through the guide rods. By means of the oil scraper ring 73 theremum allowable relative motion between housing 63 and compressing tool 74. In place of the bumper ring several bumper springs distributed along the perimeter of the. compressing tool 74 can be used.

FIGS. 9 and 10 show vertical and horizontal crosssections respectively of another design of the vibration tamper, corresponding to the system shown in FIGS. 2 and 6 with separate supporting and connecting springs. The guide sleeves 82 are located inside the projecting housing 81. Plunger disc 84 is provided with guide sleeves 85, and guide rods 87 and 88 are firmly anchored in the compressing tool. The rods 87 glide in the sleeves 82 and the rods 88 in the sleeves 85. Springs 89 and 90 hold housing 80 against the compressing tool 86. Springs 91 and 92 provide the connection between the plunger disc 84 and the acting part 86, so that all the parts involved in the oscillation process can move vertically in relation to each other.

As it can be seen from FIGS. 8 and 10, the guide rods 64, 88 and 87 are arranged in groups of three in such a manner that three rods of each group form an equilateral triangle in each corner of which a rod is located.

As shown in FIG. 9, the type of construction having separate supporting and connecting springs makespossible a unit with a low center of gravity, since the supporting and connecting springs are arranged next to each other on separate guide rods instead of on top of one another as in the device in FIG. 7. In all other respects the description in FIG. 9 corresponds to the device shown in FIG. 7.

Effective lubrication of the device covered by this invention can be achieved by installing a small piston pump in the oil sump 75 (FIG. 7). The relative motion between the plunger disc 70 and the compressing tool 74 can be utilized to drive the pump piston for the compression stroke either directly or indirectly through the plunger disc, while the intake stroke of the piston can be produced by a return spring. In this way it is possible to do without a connection between the plunger disc 70 and the pump piston. The lubricating oil thus pumped should reach all the important points to be lubricated through one or more pressure lines which can be placed at about the level of the retainers and attached there. This system of lubrication allows that all points be lubricated, including the gear drive, and the crank drive can be reached without disturbing effects from the relative motion of the parts of the device. It also prevents unnecessary damping or back thrusts caused by this damping of the driving members, which would lower the efificiencty of the preformance. The oil pump can be set in such a way that only a few cubic cemtinmeters of oil are released per stroke.

Various changes in the size, shape and arrangement of the sundry parts may be made without departing from the spirit of the invention. Hence, the appended claims should be liberally construed in a fashion comturning oil is caught and collected in the oil sump 75.

Bellows 76 connects housing 63 to compressing tool 74 and protects the glide surfaces of housing 63 and the compressing tool 74 against dirt from the outside. A bumper ring 77 is provided in the compressing tool which absorbs the blows caused by exceeding the maximensurate with the achievements set forth in the foregoing specification.

I claim:

I A tamping machine comprising:

a. a body, b. a motor mounted upon said body,

c. transmission means driven by said motor,

d. a rotatable crank driven by the transmission means, e. a plunger disc disposed below said body,

f. a linkage connected at one end to said crank and connected at the opposite end to said plunger disc for translating the rotary movement of said crank into reciprocating vertical movement of said plunger disc,

g. a tamping foot disposed below said plunger disc,

h. first spring means connected between said foot and said disc for urging said tamping foot to respond to the movement of said plunger disc,

i. a large housing encasing the connecting linkage and plunger disc, said large housing opening downwardly to receive therein the upper end of the tamping foot,

j. elongated guide rods extending in a vertical plane through the interior of the large housing, and

k. second spring means operatively associated at the one end with the plunger disc and tamper foot and connected at the opposite end to said housing for damping the shocks created when the tamper foot strikes the material to be compressed.

2. The tamping machine as defined in claim 1 wherein said second spring means is connected between said plunger disc and said housing for indirectly transmitting shock forces to said housing.

3. The tamping machine as defined in claim 1 wherein said second spring means is connected between said tamper foot and said housing for directly transmitting shock forces to said housing.

4. The tamping machine as defined in claim 1 wherein said second spring means comprises a plurality of pairs of springs, each pair being concentrically mounted about one of said elongated guide rods, one spring in each pair exerting a biasing force in opposition to the biasing force exerted by the other spring in each pair.

5. The tamping machine as defined in claim 1 wherein said first spring means are concentrically mounted upon the lower ends of said elongated guide rods.

6. The tamping machine as defined in claim 1 wherein said plunger disc has apertures formed therein for permitting the elongated guide rods to pass therethrough, whereby said disc can be moved vertically along said rods within the large housing.

7. The tamping machine as defined in claim 1 wherein said large housing further includes projections on its inner wall extending horizontally inwardly toward the interior of the housing, one of said projections being provided for each one of said plurality of elongated guide rods, each projection having an aperture formed therein, and a plurality of sleeves, one sleeve positioned within each aperture in each projection to permit one of said plurality of guide rods to pass freely therethrough in a vertical direction and to mount said second spring means on said guide rods.

8. The tamping machine as defined in claim 1 wherein said tamping foot has a cavity defined in its upper surface, said machine further including: a small housing secured to the underside of said plunger disc, said housing having an aperture in its lower wall, said housing being sufficiently small so as to be received within said cavity when the plunger disc is moved rela-' tive to the tamping foot,

a relatively short guide rod extending vertically upwardly from the center of the cavity on said tamping foot, said guide rod extending into the interior of said housing through said aperture, and having an abutment at its upper free end, and

third spring means disposed concentrically about said short guide rod between said abutment and the lower wall of said housing, said third spring means acting in opposition to said first spring means.

9. The tamping machine as defined in claim 1 wherein said tamping foot comprises a top wall, a bottom wall that engages the material to be compacted, and a sidewall that joins said top and bottom walls, said machine further comprising:

a cylindrical housing depending from the top wall of the tamping foot into the interior thereof, said plunger disc being located within said housing,

said top wall of said cylindrical housing having an aperture formed therein to permit the linkage to pass therethrough and move said plunger disc within said housing.

10. The tamping machine as defined in claim 9 wherein said first spring means is positioned within said cylindrical housing between the plunger disc and the bottom of said small housing, and said machine further includes:

third spring means positioned within said housing between said plunger disc and the upper wall of said small housing, said third spring means being positioned about the part of said linkage that extends through the aperture into the interior of said housing, and said third spring means acting in opposition to said first spring means. 

1. A tamping machine comprising: a. a body, b. a motor mounted upon said body, c. transmission means driven by said motor, d. a rotatable crank driven by the transmission means, e. a plunger disc disposed below said body, f. a linkage connected at one end to said crank and connected at the opposite end to said plunger disc for translating the rotary movement of said crank into reciprocating vertical movement of said plunger disc, g. a tamping foot disposed below said plunger disc, h. first spring means connected between said foot and said disc for urging said tamping foot to respond to the movement of said plunger disc, i. a large housing encasing the connecting linkage and plunger disc, said large housing opening downwardly to receive therein the upper end of the tamping foot, j. elongated guide rods extending in a vertical plane through the interior of the large housing, and k. second spring means operatively associated at the one end with the plunger disc and tamper foot and connected at the opposite end to said housing for damping the shocks created when the tamper foot strikes the material to be compressed.
 2. The tamping machine as defined in claim 1 wherein said second spring means is connected between said plunger disc and said housing for indirectly transmitting shock forces to said housing.
 3. The tamping machine as defined in claim 1 wherein said second spring means is connected between said tamper foot and said housing for directly transmitting shock forces to said housing.
 4. The tamping machine as defined in claim 1 wherein said second spring means comprises a plurality of pairs of springs, each pair being concentrically mounted about one of said elongated guide rods, one spring in each pair exerting a biasing force in opposition to the biasing force exerted by the other spring in eAch pair.
 5. The tamping machine as defined in claim 1 wherein said first spring means are concentrically mounted upon the lower ends of said elongated guide rods.
 6. The tamping machine as defined in claim 1 wherein said plunger disc has apertures formed therein for permitting the elongated guide rods to pass therethrough, whereby said disc can be moved vertically along said rods within the large housing.
 7. The tamping machine as defined in claim 1 wherein said large housing further includes projections on its inner wall extending horizontally inwardly toward the interior of the housing, one of said projections being provided for each one of said plurality of elongated guide rods, each projection having an aperture formed therein, and a plurality of sleeves, one sleeve positioned within each aperture in each projection to permit one of said plurality of guide rods to pass freely therethrough in a vertical direction and to mount said second spring means on said guide rods.
 8. The tamping machine as defined in claim 1 wherein said tamping foot has a cavity defined in its upper surface, said machine further including: a small housing secured to the underside of said plunger disc, said housing having an aperture in its lower wall, said housing being sufficiently small so as to be received within said cavity when the plunger disc is moved relative to the tamping foot, a relatively short guide rod extending vertically upwardly from the center of the cavity on said tamping foot, said guide rod extending into the interior of said housing through said aperture, and having an abutment at its upper free end, and third spring means disposed concentrically about said short guide rod between said abutment and the lower wall of said housing, said third spring means acting in opposition to said first spring means.
 9. The tamping machine as defined in claim 1 wherein said tamping foot comprises a top wall, a bottom wall that engages the material to be compacted, and a sidewall that joins said top and bottom walls, said machine further comprising: a cylindrical housing depending from the top wall of the tamping foot into the interior thereof, said plunger disc being located within said housing, said top wall of said cylindrical housing having an aperture formed therein to permit the linkage to pass therethrough and move said plunger disc within said housing.
 10. The tamping machine as defined in claim 9 wherein said first spring means is positioned within said cylindrical housing between the plunger disc and the bottom of said small housing, and said machine further includes: third spring means positioned within said housing between said plunger disc and the upper wall of said small housing, said third spring means being positioned about the part of said linkage that extends through the aperture into the interior of said housing, and said third spring means acting in opposition to said first spring means. 